1. Field of the Invention
The present invention relates to a capacitance detecting apparatus and its inspecting method as well as a fingerprint checking apparatus, particularly to a capacitance detecting apparatus preferably used as a fingerprint detecting apparatus and its inspecting method as well as a fingerprint checking apparatus using the detecting apparatus.
2. Description of Related Art
As a fingerprint detecting apparatus, there has been known a method in which detection electrodes are arranged in an array on a surface of a semiconductor and as shown by FIG. 17, when a finger is put on an overcoat 102 covering the detection electrodes 101, capacitances Cs formed in accordance with recesses and projections of a fingerprint between the detection electrodes 101 and a surface of the finger, are detected to thereby sample a pattern of a fingerprint (fingerprint pattern) (for example, refer to U.S. Pat. No. 5,325,442).
According to the capacitance Cs formed between the detection electrode 101 and the surface of the finger, a capacitance value is increased at a portion of a ridge or the fingerprint since a distance between the ridge portion and the detection electrode 101 is shortened, the capacitance is reduced at a portion of a valley of the fingerprint since a distance between the ridge portion and the detection electrode 101 is lengthened and accordingly, the pattern of the finger print can be sampled by detecting the capacitance Cs. As a method of detecting the capacitance Cs, there are conceivable two methods of a current charge method and a voltage charge method.
The former current charge method is a method in which after flowing constant current Ic during a constant time period of Tc from the detection electrode 101, that is, after charging constant electric charge xcex94Q to the detection electrode 101, a voltage change xcex94V of the detection electrode 101 is detected. The voltage change xcex94V and the capacitance Cs are in an inversely proportional relationship as is apparent: from the following equation (1).
xcex94V=xcex94Q/Cs=IcTc/Csxe2x80x83xe2x80x83(1)
The latter voltage charge method is a method in which after charging electric charge to the detection electrode 101 by constant voltage xcex94Vc, the electric charge xcex94Q is detected. The electric charge xcex94Q and the capacitance Cs are in a proportional relationship as is apparent from the following equation (2).
xcex94Q=Csxcex94Vcxe2x80x83xe2x80x83(2)
FIG. 18 shows a circuit constitution of a conventional example of a fingerprint detecting apparatus using the current charge method. In the drawing, row drive lines 111 and column sense lines 112 are wired in a matrix shape in respect of the detection electrodes 101 arranged in an array. An NchMOS transistor Q1 of a source follower and an NchMOS transistor Q2 for selecting a row are connected in series between a power source line 113 and the column sense line 112. Further, the gate of the MOS transistor Q1 is connected to the detection electrode 101 and the gate of the MOS transistor Q2 is connected to the row drive line 111, respectively.
Further, a PchMOS transistor Q3 and a charge current source Ic are connected in series between the power source line 113 and the ground. Further, the gate of the MOS transistor Q3 is connected to a reset line 114. Further, a common connection point P for connecting the MOS transistor Q3 and the charge current source Ic is connected to the detection electrode 101 via an NchMOS transistor Q4. Further, the gate of the MOS transistor Q4 is connected to a charge control line 115.
The circuit having the above-described constitution is provided to each of the detection electrodes 101, that is, the unit cell. Here, an explanation will be given of operation of the circuit in reference to timing charts of FIG. 19.
First, the MOS transistor Q2 is brought into an ON state by being applied with a row drive signal RAD at a high level (hereinafter, described as xe2x80x9cHxe2x80x9d level) via the row drive line 111 and successively, the MOS transistor Q4 is brought into an ON state by being applied with a charge control signal CEN at xe2x80x9cHxe2x80x9d level via the charge control line 115. Thereby, selection of row is carried out.
Simultaneously with the row selection, the MOS transistor Q3 is brought into an ON state by being applied with a reset signal XRST of a low level (hereinafter, described as xe2x80x9cLxe2x80x9d level) via the reset control line 114. Thereby, voltage (hereinafter, referred to as detection voltage) Vs of the detection electrode 101 is reset to power source voltage VDD which is reference voltage. Thereafter, by transition of the reset signal XRST to xe2x80x9cHxe2x80x9d level, the MOS transistor xe2x80x9cQ3xe2x80x9d is brought into an OFF state. Thereby, electric charge produced by the current source Ic starts charging to the detection electrode 101 via the MOS transistor Q4.
After elapse of the constant time period Tc, the charge control signal CEN transits to xe2x80x9cLxe2x80x9d level by which the MOS transistor Q4 is brought into an OFF state. Thereby, the electric charge finishes charging to the detection electrode 101. A change amount V at this occasion from resetting the detection voltage Vs is given by Equation (1). The detection voltage Vs is read by the row column sense line 112 via the MOS transistor Q1 of the source follower and the NchMOS transistor Q2 for selecting the row and is outputted to outside via the column sense line 112.
As described above, according to the conventional fingerprint checking apparatus using the current charge method, by detecting the voltage Vs of the detection electrode 101 produced by charging the constant electric charge xcex94Q, the capacitance Cs formed between the detection electrode 101 and the surface of the finger can be detected. However, the detection voltage Vs is constituted to output via a plurality of transistors or in the case of this example, via the MOS transistors Q1 and Q2 and accordingly, there poses a problem in which S/N of the detection signal is deteriorated by a dispersion in characteristics of these transistors such as a threshold value Vth or ON resistance in respective cells.
Further, in order to efficiently sense the detection voltage Vs of the respective detection electrode 101, there are needed a plurality of the charge current sources Ic having the same current value (in the case of this example, the charge current sources Ic are prepared for the respective cells) and therefore, a dispersion in current values of these current sources Ic constitute one factor of deteriorating S/N.
Further, the voltage Vs must be sampled while maintaining the electric charge of the detection electrode 101 and accordingly, a source follower circuit (in the case of this example, MOS transistor Q1) needs to use, the gate of the MOS transistor Q1 is connected to the detection electrode 101 and accordingly, there is a concern of causing electrostatic breakdown at the gate portion when, for example, a charged finger is put thereon.
In the meantime, in the case of the voltage charge method, switching elements of a number of rows for selecting the rows are connected to respective column sense lines by a number of rows and accordingly, relative to the capacitance Cs to be sensed, parasitic capacitance Cs1p of the column sense line for sampling thereof is very large and therefore, in order to sample electric charge which is charged to the capacitance Cs, some devise is needed.
As an example, in the constitution shown by FIG. 17, when a size of the detection electrode 101 is set to 80 xcexcmxc3x9780 xcexcm, a material of the overcoat 102 is SiN and its thickness is set to 1.0 xcexcm, assuming the specific inductive capacity of SiN as 7.5, a maximum value Cs (MAX) of the capacitance Cs becomes 425 (fF).
In contrast thereto, when a number of detection rows is set to 128, parasitic capacitance of the switching element to be connected is set to 5 (fF) and parasitic capacitance of wirings is set to 0.4 (pF/mm), the parasitic capacitance Cs1p of the column sense line is given as follows.                     Cs1p        =                              128            ·            0.005                    +                      0.08            xc3x97                          128              ·              0.4                                                              =                  4.74          ⁢                      xe2x80x83                    ⁢                      (            pF            )                              
Therefore, the column sense line is attached with the parasitic capacitance Cs1p ten times as much as the capacitance Cs to be detected or more (Cs1p/Cs greater than 10).
The present invention has been carried out in view of the above-described problem and it is an object of the present invention to provide a capacitance detecting apparatus resolving the problem of the current charge method by using the voltage charge method and capable of firmly sampling electric charge charged to capacitances by a simple circuit constitution, its inspecting method as well as a fingerprint checking apparatus using the detecting apparatus as a fingerprint detecting apparatus.
According to an aspect of the present invention, there is provided a capacitance detecting apparatus comprising unit cells having detection electrodes and switching elements connected between the detection electrodes and sense lines and arranged in an array shape, charging and discharging circuits for charging electric charge to the detection electrodes under constant voltage and discharging the electric charge and detecting circuits for imaginarily grounding the sense lines after charging the electric charge to the detection electrodes and detecting the electric charge of the detection electrodes via the sense lines to thereby detect capacitances formed between a detection object and the detection electrodes.
According to the capacitance detecting apparatus having the above-described constitution, the charging and discharging circuits charge electric charge to the detection electrodes of the unit cells under constant voltage. After the charging operation, the detecting circuits imaginarily ground the sense lines. By the imaginarily grounding operation, detected voltage detected by the detecting circuits via the sense lines, is not dependent on characteristics of the switching elements connected to the detection electrodes for the respective cells and a dispersion thereof is reduced. Further, the detecting circuits output the detected voltage having small dispersion as a result of detecting the capacitances formed between the detection object and the detection electrodes.
According to another aspect of the present invention, there is provided a capacitance detecting apparatus comprising unit cells having detection electrodes and switching elements connected between the detection electrodes and sense lines and arranged in an array shape, detecting means for detecting capacitances formed between a detection object and the detection electrodes by charging electric charge to the detection electrodes and detecting voltage based on the electric charge and dummy electrodes having parasitic capacitances substantially equal to parasitic capacitances of the detection electrodes, wherein electric charge of the parasitic capacitances of the detection electrodes is canceled by electric charge of the parasitic capacitances of the dummy electrodes.
According to the capacitance detecting apparatus having the above-described constitution, the parasitic capacitances are provided between the detection electrodes and a substrate. Further, when electric charge is charged to the detection electrodes, the electric charge is charged not only to the capacitances formed between the detection object and the detection electrodes but also to the parasitic capacitances of the detection electrodes. The electric charge of the parasitic capacitance of the detection electrode constitutes DC offset when the capacitance is sensed. In contrast thereto, by providing the dummy electrodes having the parasitic capacitances substantially equal to the parasitic capacitances of the detection electrodes, electric charge to the same degree of electric charge charged to the parasitic capacitances of the detection electrodes is charged also to the parasitic capacitances of the dummy electrodes. Accordingly, by utilizing electric charge the parasitic capacitances of the dummy electrodes, electric charge of the parasitic capacitances of the detection electrodes can be canceled. As a result, there is produced no DC offset caused by the parasitic capacitance of the detection electrode.
According to an inspecting method of a capacitance detecting apparatus of the present invention, in the respective capacitance detecting apparatus having the above-described constitution, attention is paid to the fact that when, for example, the finger is not put on the detection electrode, capacitance is not formed between the finger and the detection electrode, after charging electric charge to the detection electrode under constant voltage, electric charge of parasitic capacitance between the detection electrode and a substrate is read and the acceptability of a switching element is confirmed based on the read electric charge of the parasitic capacitance. Further, in the case of a capacitance detecting apparatus having dummy electrodes, in confirming the acceptability of the switching elements, a canceling function based on the parasitic capacitances of the dummy electrodes is stopped.
In the case in which, for example, the finger is not put on the detection electrodes, when electric charge is charged to the detection electrodes under constant voltage, the electric charge is accumulated only to the parasitic capacitances between the detection electrodes and the substrate. After the charging operation, the electric charge of the parasitic capacitances is read out. Thereby, even when the finger is not put on the detection electrodes, by reading the electric charge of the parasitic capacitances, the acceptability of the switching elements, that is, whether the switching elements operate normally can be confirmed. In the case of the capacitance detecting apparatus having the dummy electrodes, by stopping the canceling function, the acceptability of the switching elements can be confirmed without undergoing influence of the parasitic capacitances of the dummy electrodes.
A fingerprint checking apparatus according to the present invention uses the respective capacitance detecting apparatus having the above-described constitution as fingerprint detecting means. Further, the fingerprint checking apparatus includes storing means for storing pattern data of a fingerprint which has previously been registered and comparing means for comparing fingerprint data detected by the fingerprint detecting means with registered pattern information stored to the storing means and outputting a result of the comparison as a fingerprint checking result.
According to the fingerprint checking apparatus having the above-described constitution, firstly, pattern data of a fingerprint constituting a checking object is previously stored to the storing means as registered data. Further, the comparing means compares detected finger print data with the previously registered pattern data and when they coincide with each other, the comparing means determines that the detected fingerprint is the previously registered fingerprint and outputs a checking result stating the determination.